The Challenge and Future Development of Hydraulic Fracturing in Deep Hot-Dry Rock in EGS

Hydraulic fracturing creating an artificial fractures or establishing a fracture network is an effective technology to conduct fluids in multiwell enhanced geothermal system (EGS). Rock mechanics tests show that deep hot-dry rock (HDR) always has high strength and in-situ stress, the traditional hydraulic fracturing concept of form bi-wing artificial fractures will face enormous challenges in deep HDR. Mini-fracturing test and main-fracturing reveal that the tensile failure not only leads to an ultra-high pumping pressure, but also limits the stimulation radius. Natural fracture is one of the most important parameters controlling the rock failure mode, increasing the injectivity index and reducing the injection pressure. Hybrid tensile failure is the most likely failure mode for deep HDR hydraulic fracturing, which includes the shearing and propagating of nature fractures and increases the conductivity and radius of the fracture network. The elasto-plastic constitutive model reveals the recoverable elastic strain and irreversible plastic strain mechanism, and characterizes the mechanical aperture change of rough surface of nature fractures. The effective conductivity aperture of natural fractures affected by combined effect of heat transfer, fracture fluid flow, shearing dilation and chemical dissolution. Therefore, the challenge and future development of hydraulic fracturing in deep dry-hot rock in EGS is the coupling of Thermal-Hydro-Mechanical-Chemical (THMC).